From support.apple.com :
In some situations, you need to enter your password instead of using Touch ID:
- If you've just restarted your Mac
- If you've logged out of your user account
- If your fingerprint isn't recognized five times in a row
- If you haven't unlocked your Mac in more than 48 hours
- If you've just enrolled or deleted fingerprints
If you still need help unlocking your Mac with Touch ID, follow these tips. Try again after each one:
- Make sure that your fingers are clean and dry.
- Moisture, lotions, sweat, oils, cuts, or dry skin might affect fingerprint recognition. Certain activities can also temporarily affect fingerprint recognition, including exercising, showering, swimming, or cooking.
- Your finger should cover Touch ID completely. While Touch ID is scanning, don't press down or move your finger.
- Try setting up a different fingerprint.